Gravity is considered to be a three-dimensional vector force. Utilizing an X, Y and Z-coordinate system, gravity can readily be defined as an attractive force between an object and the earth with the force having components in all three dimensions. If the coordinate system is conveniently defined at the center of the earth and the Y-axis is defined along the line between the earth and the object of interest, then the Y-component of gravity will be quite large compare to the X and Z-components. If they are conveniently defined as the north-south and east-west components (bringing into play the relatively well known surface coordinates on the earth), then those components are materially smaller. It will be appreciated that measurement of the Y or large component of gravity is extremely significant in certain scientific phenomena.
One phenomena where gravity measurements of the earth are extremely helpful is in prospecting for minerals. The earth is not a homogeneous body. As a result, it is known that pattern variations in the measurement of the vertical component of gravity over a given geological region may very well show a set of variations which are coherently related to the geology of the region. As an example, large masses of iron ore create regional discontinuities in the measurements which, on proper interpretation, yield valuable information for determining the extent of the mass of iron ore in the earth.
While regional variations in gravity occur, variations also occur at a given locale over long or short periods of time as a result of a variety of reasons including, as an example, movement of extraterrestrial bodies. Accordingly, a set of base measurements over a period of time are usually deemed necessary to have a fixed base measurement whereby mesurements taken in a large locale (for instance, in prospecting for various mineral deposits) are made so that all measurements can be referenced (by subtraction of time variations) to a common base station measurement to obtain time invariant measurements. To the extent that measurements at a given spot vary over a time interval, such variations are mathematically removed for the purpose of achieving a base station measurement taken in the locale. The present invention is a gravity meter which responds to variations in the vector component of gravity acting between the gravity meter and the earth and which converts such variations into a physical movement which can be measured and recorded on a time base chart.
The present invention has as one of its advantages a gravity measuring system using a hydraulically damped mass which mass moves in response to gravity variations. Such damping fairly well eliminates instrument system induced variations as might occur with an undamped structure. The mass moves responsive to variations over a period of time with sufficient damping so that overshoot, oscillations or transducer errors are not induced. The apparatus achieves this by utilizing a gravity attracted mass in a liquid bath. The system is balanced by supporting the transducer mass from a horizontal beam of significant length, one end of the beam being mounted on a pivot mount to enable the beam to deflect. The beam is thus rotated around its pivot by the transducer mass. The slightly arcuate movement of the transducer mass as it rotates around the pivot point of the mounting beam is an excursion of only a few microns, and, therefore, angular distortion of the response is minimal. The beam and hydraulic damping system which receives the transducer mass is relatively simple in structure and is, therefore, relatively straightforward in mounting. This is particularly advantageous in initially setting up the equipment and adjusting it to maximum sensitivity by eliminating off-balance mounting and other distorting forces.
An important feature of this apparatus is the multiplier which is incorporated. The multiplier is an apparatus which connects with the horizontal beam and enlarges movement thereof. The multiplier converts the relatively small movement of the transducer mass into a much larger movement by a scale factor of between approximately 100 to 1,000. This scale factor is controlled by sizing of the multiplier. Accordingly, it can be varied to a requisite value for the purpose of obtaining a different excursion in response to the transducer mass. The multiplier has as one feature the incorporation of a indicator disk mounted at its remote upper end. The indicator disk is observed in location by utilization of a photoelectric sensor and light bulb, thereby coupling movements of the indicator disk to a recording instrument.
From the foregoing, it will be understood how the apparatus is able to respond to variations in the vertical component of gravity which are converted into excursions of significant amplitude. They are converted and placed in a form enabling recordal on a time base chart mechanism.